At first glance, the brain and the gut do not have a lot in common, but in reality, they work closely together through an exchange of hormonal, immune, and nervous signals.
The ecosystem of microorganisms that live in the human gut plays an important role in this relationship.
For example, research in mice suggests that a significant reduction in the diversity of gut bacteria, or an increased number of disease-causing species may lead to depression.
At the same time, there is some evidence that probiotics, which introduce friendly bacteria into the gut, can improve symptoms of depression.
A recent review of the research to date, reported by Medical News Today, concludes that probiotics alone or in combination with prebiotics — which feed the friendly bacteria that are already in the gut — may be effective treatments for depression.
However, the same review also notes there is insufficient evidence that these supplements have a beneficial effect on people with anxiety.
Nevertheless, some research does hint that prebiotics may help otherwise healthy people cope with stress, and that they can reduce milder forms of anxiety.
For example, a 2014 study found that a type of prebiotic supplement called galacto-oligosaccharides (GOS) reduced healthy adults’ production of the stress hormone cortisol and improved their emotional processing skills.
GOS are a type of carbohydrate that is indigestible. This means that they reach the intestine intact, where they can feed the microbiota that live there.
On the basis of the earlier study, psychologists at the University of Surrey in Guildford in the United Kingdom investigated whether GOS could be beneficial to the long-term mental health of adolescent females on the threshold of adulthood.
The transition from adolescence to adulthood is a key developmental stage for the emergence of a person’s ability to regulate emotions such as fear and anxiety.
The present research, which appears in the journal Scientific Reports, suggests that the supplement may reduce anxiety in individuals with the highest trait anxiety levels.
It also found a small but noteworthy increase in the abundance of a genus of bacteria called Bifidobacterium in the gut of those individuals, which may explain this change.
“This new research marks a significant step forward in that we were able to show that we can use a simple and safe food supplement such as prebiotics to improve both the abundance of beneficial gut bacteria in the gut and to improve mental health and well-being in young women,” says Dr. Kathrin Cohen Kadosh, senior author of the study.
GOS are found naturally in dairy products, beans, and certain root vegetables.
Medical News Today asked Dr. Cohen Kadosh whether poor diet in adolescence and young adulthood could affect anxiety levels due to its influence on the gut microbiota.
“The GOS dietary supplement in our study led to an increase in beneficial gut bacteria, which in turn affected behavior and well-being. Based on this, it is plausible to suggest that a diet rich in GOS could have similar effects outside the lab; we already know that this is the case in animals.”
Dr. Cohen Kadosh added that this could be particularly important during a formative period, when behavioral patterns and the underlying brain networks develop continuously and become established.
Fine-tuning the gut-brain axis?
In their paper, the psychologists write that in animals, adolescence is a critical time when gut microbiota help fine-tune the gut-brain axis.
To test their hypothesis that this may also be the case for humans, they recruited 64 healthy female volunteers aged 18–25 years.
The participants had no current or previous diagnoses of anxiety, were not taking any prebiotic or probiotic supplements, and had not used antibiotics for the previous 3 months.
As part of the study, they took either a GOS supplement or a placebo daily for 4 weeks.
At the start and end of the study period, they filled out standard questionnaires for assessing anxiety, depression/mood, and emotional regulation abilities.
They also performed an attentional dot-probe task, which is a test of a person’s emotional bias toward positive or negative stimuli.
In addition, the participants provided a stool sample at the start and end of the study to allow the researchers to assess changes in the composition of their gut microbiota.
Out of 64 participants who started the study, 48 completed it.
High and low anxiety groups
Overall, there were no apparent effects of the supplement on self-reported levels of anxiety, bias toward positive emotional stimuli in the dot-probe task, or the abundance of beneficial gut bacteria.
However, when the researchers divided participants into high and low anxiety groups based on their reports at the start of the study, some interesting trends began to emerge.
There were reductions in anxiety in the high anxiety group who took GOS compared with the high anxiety group that took the placebo.
The participants in the high anxiety group also showed a reduced bias to negative stimuli, and increased bias to positive stimuli in the dot-probe task.
These participants also had significantly greater increases in the abundance of Bifidobacterium in their gut.
The main limitation of the study was that there was a 25% dropout rate among participants. This means that numbers in the high and low anxiety groups were mostly too small for the researchers to draw statistically significant conclusions.
The researchers say larger studies will be needed to confirm their results. However, evidence is mounting regarding prebiotics and probiotics.
MNT spoke with Georgios Argyros, a Ph.D. student from the University of Edinburgh, who said: “The evidence suggests that pre/probiotics have a promising future as a therapeutic approach for depression and anxiety. If we look at the latest meta-analyses, there is an indication that [pre/probiotics] could alleviate symptoms of both depression and anxiety. Nonetheless, more research is required to establish their efficacy and effectiveness.”